1. Field of Invention
Embodiments of the present invention relate generally to a web lift system and a method for lifting a web in a polishing system.
2. Background of Invention
In semiconductor wafer processing, the use of chemical mechanical planarization, or CMP, has gained favor due to the enhanced ability to increase device density on a semiconductor workpiece, or substrate, such as a wafer. As the demand for planarization of layers formed on wafers in semiconductor fabrication increases, the requirement for greater system (i.e., process tool) throughput with less wafer damage and enhanced wafer planarization has also increased.
An exemplary CMP system that addresses these issues is described in U.S. patent application Ser. No. 09/244,456, filed Feb. 4, 1999 to Birang et al., which is incorporated by reference in its entirety. Birang et al. disclose a CMP system having a planarization system that is supplied wafers from cassettes located in an adjacent liquid filled bath. A transfer mechanism, or robot, facilitates the transfer of the wafers from the bath to a transfer station. The transfer station generally contains a load cup that positions wafers into one of four processing heads mounted to a carousel. The carousel moves each processing head sequentially over the load cup to receive a wafer. As the processing heads are loaded, the carousel moves the processing heads and wafers through the planarization stations for polishing. The wafers are planarized by moving the wafer relative to a polishing material in the presence of a slurry or other polishing fluid medium. The polishing material may include an abrasive surface. The slurry typically contains both chemicals and abrasives that aid in the removal of material from the wafer. After completion of the planarization process, the wafer is returned back through the transfer station to the proper cassette located in the bath.
Conventional polishing pads are generally comprised of a foamed polymer having a textured or porous surface. The textured or porous surface functions to retain the polishing fluid that normally contains abrasive slurry on the polishing pad during the polishing operation. The abrasives in slurry provide the mechanical component of the planarization process planarizes (i.e., polishes) the substrate in concert with chemical agents present in the polishing fluid.
One type of polishing material that may be utilized for chemical mechanical polishing is known as a fixed abrasive material. The fixed abrasive material comprises a plurality of abrasive particles suspended in a resin binder that is disposed in discrete elements on a backing sheet. As the abrasive particles are contained in the polishing material itself, systems utilizing fixed abrasive material generally use polishing fluid that do not contain abrasives. Such polishing fluids enhance the service life of their fluid delivery systems.
Both conventional and fixed abrasive polishing material are generally available in stick-down form or in the form of a web. Generally, conventional polishing material may loose ability to adequately retain polishing fluid over the course of polishing a number of substrates as the polishing surface of the material is consumed by the polishing process.
Fixed abrasive material is typically used in web form. Generally, the polishing process wears down the abrasive elements disposed on the web. To maintain a polishing surface that produces uniform polishing results, the web is periodically indexed to remove portions of the web that may have become worn, replacing those portions with an unused portion of the web.
However, indexing the web across a polishing platen is sometimes difficult. The polishing and other fluids that come in contact with the web may cause surface tension or attraction to develop between the web and the underlying surface of the platen. This surface tension must be over-come to accomplish advancement of the web. If the attraction between the web and platen is great, the indexing means may not be able to index the web or the web may become damaged during the indexing process.
Providing a cushion of gas between the web and platen assists in overcoming the attraction between the web and platen. The gas lifts the web to a spaced-apart relation to the platen where the web may be freely indexed. However, providing gas to the area between the web and platen is complicated, and requires rotary union and process tubing to be routed through an already crowded platen.
Therefore, there is a need for a system that lifts a web of polishing material from a platen so that the web may be freely moved across the platen.